Joint assemblies are most often found between adjacent panels or sections of transportation and building structures such as pedestrian walkways (e.g., sidewalks), highways, airport runways, bridges, parking garages, parking lots, architectural and building (exterior and interior) facades, roofing, gutters, and other similar structural elements. Joint assemblies are designed and installed to (1) enable the adjacent panels (or sections) to expand and contract without cracking, (2) prevent water, debris, and the like from entering between (and thereby causing deterioration of) the adjacent panels, and (3) maintain a durable, smooth, substantially continuous surface between the adjacent panels being joined by the joint assembly. Proper installation of joint assemblies therefore provides many benefits including but not limited to ‘sealing’ the structures from the elements and reducing heating and cooling costs. Because of these benefits, joint assemblies are widely used in modern construction.
A joint assembly is usually filled with a sealant composition capable of adhering to the walls of the adjacent structural panels (“adhesion” or “adhesive strength”) and of withstanding at least the expected recurring movement of adjacent structural panels relative to one another for a given period of time (“expected joint assembly movement”). Joint assembly movement is typically caused by thermal expansion, but other factors including but not limited to swelling with moisture, wind sway, and vibrations, contribute to the expected joint assembly movement.
The “movement ability” of the sealant composition allows the sealant composition to absorb the expected joint assembly movement. If a sealant composition has a small movement ability, then the joint assembly panels have to be smaller and more joint assemblies will be needed so that the expected joint assembly movement does not exceed the sealant's movement ability. The movement ability is typically expressed as a percentage of the joint assembly width. In contrast, when a sealant composition has a larger movement capability, buildings can be designed with larger panels, or smaller more attractive joint assemblies, or construction variances can be larger and more forgiving, making construction faster and more economical.
Sealant compositions typically have a gelatinous-type consistency (at least initially) which permits easy application between adjacent panels. Preferably, the sealant composition cures in-situ to form an elastomeric-type material. A wide variety of sealant compositions have been developed, including silicone sealant compositions, butyl rubber sealant compositions, acrylic sealant compositions, urethane sealant compositions, and modified urethane sealant compositions. Such sealant compositions generally include a polymer having a molecular weight low enough for ease of application and a curing agent which causes cross-linkages to form between the low molecular weight polymers (preferably, after application of the sealant composition), thereby resulting in the formation of a cross-linked/branched polymeric material in-situ.
In most joint assemblies, the cohesive strength of a sealant composition is not an important factor, particularly in pure sealing applications. In fact, high sealant composition cohesive strength can be detrimental if the surfaces of the adjacent panels being joined by the joint assembly are weak.
On the other hand, however, high sealant composition strength (adhesive and cohesive) is desirable in structural glazing applications where the sealant composition is essentially a flexible glue which both holds the glass panel in a building (or other structure) and keeps the weather out. Given the environmental conditions (e.g., high winds and large temperature changes) encountered in many structural glazing applications, at least some joint assembly movement capability is required.
Modulus is the ratio of stress to strain, and a lower modulus sealant composition with higher movement ability is generally more desirable than higher cohesive strength in joint assembly applications. As a joint assembly moves, the sealant composition in the joint assembly is stretched and exerts a force on the bond line (of the adjacent panels). If the sealant composition is too stiff, the force created by a large movement will be large, and thus weak surfaces can be pulled apart and strong surfaces often see a bond break since the force created will be greater then the adhesive strength of the sealant composition bond to the panel. Conversely, the adjacent panel surfaces of joint assemblies which include lower modulus sealant compositions experience less stress during joint movement. If the sealant composition has a large movement ability and exerts a very low force with movement (“a low modulus, high elongation sealant composition”), the sealant composition will have a better chance of successfully handling the movement and keeping the bond intact. Moreover, joint assemblies including lower modulus, high elongation sealant compositions often are able to satisfy any required joint movement for relatively greater periods of time because these products produce less fatiguing stress (i.e., less force is created during joint assembly movement).
Despite the widespread application of joint assemblies, however, their importance is often not fully appreciated -unless joint assembly failure occurs. Joint assembly failure often causes property damage, and is often attributed to joint assemblies lacking in durability, i.e., joint assemblies which cease to be effective after a short period of time. For example, when the joint assemblies in a building structure (such as concrete panels) deteriorate such that they cannot prevent water from entering, the interior walls of a building become discolored and/or the building contents become soiled. Such deteriorated building structure joint assemblies can also significantly increase heating and cooling costs. Water ingress through failed joint assemblies is also an important cause of mold growth and sick building syndrome. Additionally, when the joint assemblies between adjacent concrete slabs of a transportation structure (such as a highway) fail, they often do so by allowing debris (incompressibles) to enter. Water can then enter the transportation joint assembly and get below the slabs, deteriorate the base (e.g., sand), and cause the slab to crack and break. The deterioration and failure of such transportation structure joint assemblies often further results in gap formation between the adjacent slabs, which causes inconvenience and discomfort to vehicular and pedestrian traffic. Such transportation and building structure joint assemblies often fail because their sealant compositions do not have the requisite low modulus and movement ability to satisfy the expected joint assembly movement.
Therefore, performance improvements in joint assemblies and methods of installing same are still being sought. Similarly, improved sealant compositions for use in joint assemblies are desired.